1. Field
The invention generally relates to generating a transport data stream with picture data, and more particularly to generating transport data streams for use in digital television systems.
2. Description of Related Art
In order to transmit images for digital television (TV), typically the digitally recorded image sequence is transformed in a first processing step which performs, in part, source encoding—in this case, video encoding—into a stream of successive data frames, each of which contains image data of one image.
Currently, video encoding for digital television is defined by the MPEG4-10 (H.264/AVC) encoding and compression standard. In this context, a stream of successive data frames has a cyclically repeating sequence of several data frames, or group of pictures (GOP), that is composed of an I-frame (intra-coded frame) with all image data belonging to one image transmitted at the start of the GOP, several P-frames (predictive coded frames), and/or B-frames (bidirectional coded frames). While a P-frame contains only the altered image data of the appurtenant image relative to the image data of the image belonging to the preceding P-frame or I-frame, the altered image data of the appurtenant image relative to the image data of up to n preceding and up to n following P-frames or I-frames are deposited in the B-frame. Also, in the event of a scene change in the image sequence, an I-frame is used instead of a P-frame that reproduces the scene change better.
These structure characteristics of the GOP used in a stream of successive data frames can be parameterized in the case of the MPEG4-10 (H.264/AVC) encoding standard at the initialization of the transmission system. For the continuous operation of the transmission system, the number of data frames per GOP (i.e., the number of images per GOP) and the number of generated data frames per time unit (i.e., image repetition rate or video frame rate) can be adjusted.
In the next processing step of channel encoding, a data structure of the channel-encoded data stream is generated, starting from the GOP data structure of the stream of successive data frames. The format of the data structure depends on the digital TV transmission standard under which it is implemented and its corresponding encoding algorithm.
In the case of the U.S. digital mobile television standard, the Advanced Television Systems Committee Mobile/Handheld (ATSC-M/H) standard, for example, a stream of ATSC-M/H data frames (or simply ATSC-M/H “frames”) consisting in each case of 5 ATSC-M/H sub data frames (or simply “subframes”) is formed as shown in FIG. 2. In turn, the subframes are composed in each case of 16 ATSC-M/H time slots. The image data to be transmitted for each TV program are packed by a multiplexer in ATSC-M/H time slots provided for each TV program in each ATSC-M/H subframe and are channel-encoded with a channel encoder, preferably a Reed-Solomon encoder. An ATSC-M/H frame has a fixed length of time of, for example, 967.887 ms in accordance with the parameters of the ATSC-M/H standard listed in the table of FIG. 4.
WO 2008/092705 A2, incorporated herein by reference in its entirety, provides additional background on systems operating under the ATSC Digital Television Standard (A/53) and on providing ATSC interoperability.
As can be seen in the table shown in FIG. 5, due to the fixed length of time of the ATSC-M/H data frame in the amount of, for example, 967.887 ms, a different number of packets in an ATSC-M/H data frame that is not quantified as a whole number results for different values of the image repetition rate, or video frame rate, i.e. line 1 of the table in FIG. 5.
Differing pulsing between the image repetition rate in the GOP-structured data frame stream and the fixed transmission rate of ATSC-M/H data frames for an image repetition rate of 30 images/second is shown graphically in FIGS. 6A and 6B. In FIG. 6A or, respectively, 6B, it can be seen that in the first ATSC-M/H data frame, i.e. #0, the individual I-frames are positioned at the start of the ATSC-M/H data frame at all image repetition rates while in the following ATSC-M/H data frames, i.e. #1 through #7, they are staggered at all image repetition rates by the number of the ATSC-M/H data frames already transmitted and multiplied by the pulse rate differential from the start of the respective ATSC-M/H data frame.
If at any time a new program is selected by the viewer of digital television in which a P-frame is received, an insufficient reconstruction of the image belonging to the P-frame results, which manifests itself as artifacts in the image or in a very noisy picture due to the reduced image information of the P-frame and the lacking image information of the I-frame referenced by the P-frame. A successful source decoding will not result until after the channel decoding of the initially received I-frame. This unpredictable start of a successful source decoding due to the undetermined position of the initially received I-frame and, as a result, disadvantageously delays the entry time into a digital television program when the program is joined or in the event of a program change.